Method of processing metal powders



May l2, 1953 F. J. HANsGlRG METHOD oF PROCESSING METAL PownERs FiledFeb. 9, 194e w w M .i ,e f4 M E p W d r w M W E, M 0 w t l 7 aa Z 3 WW m3 f TM mr um n m u a E J M uv 0 f Il' V L. w Z H27 9 r ,f., z :s 3 ,0, f6 im w3 M xy Eph v l 4J 0 W n y Patented May l2, 1953 Q'FFI-CE assenze..

METHODQEPROGESSING METAL POWDERS.

February 9, 13416', Senial No., 646,554- c tion orfexnlosionplnzerdtTheims of. auch; as oxides end nitrdes, ,fermedlinltheIsurface-il.tiefereof the powdery, metal carnales y-erehierly; objectionable for manylapplteaticnseetithe powders. in-that the productsmade therefwmmraheteroseneeusdue to the inclusion fheinfihQSe-Qompund-If me nowderszam used yfor powder metallurgical y purposes the flmsoftheeompoundsstated, render it diicult to obtain a coherent compactedbodrandtefulLw-sinter it.. ',lhefemtereefhediee are hetereseneouar., orinsumeienn density; and ccusanientlsrl ct interim Rhysinalf prenantes@-It. therefore an. Ohieet cet; the inrentiento process .metal .y powders'QI aluminum-orf. meeneslum.` solas .to .remoref-suroeerfilms er.@bioeticaable compo,umtsffromr ther-rpertclesfinan-ecenomical and.eieotivo-,mnnm n It isanothembieetonthe invention tenroeees powdersofthe kind hemnswmmedt. Kl-t remore from their;martiennesunerfleteliiilrnsi-y of obieetionahle. comanda, :suelah es.oxides.: and nitrdes. andato immunise-vthe-,sorfaee lareraof thecleaneaperticles surement-,attacks by t1.1e. ncreeno1A nirwanaeeneinedinlthe surrounding, air, atleast-.for a limited mcricepofftime.

It is. further.l objecttof the., invention te; removeobjectionable-compounds., nrimrilronides and frritrdes;` formed by;Icontesa: with air infierir- Iacelayeraoifnowdery metal particles?Qffaluminum or magnesium man economical. ena-emotive Process. SuitedYfor; mais: wwwmain It-xsstill a further atleet@cffftberinrentimetoreduce thereuraoazleyerseof porselein*t nartieles. of metan hereinconcerned? te their; metallic ifstate by removing;therefrom-:..obieotimable communes premouslyf forme@ therein bs;contacta vwitlr-` air or, otherwise. andztcrstebilize 4or;rllmmvcrircagainst Subsequent attackszofmin those entferne lwersat least fon-alimited of; time; in.- an eetive ind eeonomicalmsolssauitedf;mossenroduc- These and. ethemhi eestoiathwinventimmill be morecleanse,.- undaretaodz when minut-ion proceed witlxtrefonmatmthsixxovmioh 2V Fis. lxllastrates schematically 0n hand 0f a HOW sheet alprocess according to the invention snitedafgoi; mass producton,y andFig. 2 shows scnematicallyiandin cross-section a Vacuum drying L nousable with the invention.

Acco ding tothe invention, lthe powder of a metateueh. er magnesium,prepared enr known and. Sui-table manner and the particles of? whichcontain in their surface layers objectionableffcompounds, primarilyoxides and/.or nitrideauis subjected to a chemical treatment or washing,process whichy removes the objectionablecompoiindsfrom theparticlesurfaces and leaves the surfaces clean and resistant to corrosion andother chemical reaction with oxygen 0r nitrogen contained in thesurrounding air, and atlvleastffor a` limited period of time. To thisendl according teY the invention. the metal powder is treatedwithnitricacid (NHOa) of suitable hignirconcentration so as yto remove from thepowderyparticles-tlie objectionable oxide and/or nitride filmsrapidly;this erTect manifests itself by a cleanvanl metallic, shiny appearanceof theftljeated particles.v By the action of the high- 1ycqncentratedinitric acid upon the surface layers of the Vparticles fromwhich those compounds are beingrernoyed, thoselayers are also cleaned intneaforementioned sense .so that they stayirninline againstftheattacksof oxygen-or nitrogen ysuch as contained. in the surrounding air, ortheir v t calativity with respect to that oxygen ornitigogen issubstantially reduced at least for a, limited time periodwhich sufficesfor manipulating4 or other subsequent treatment of the powder inpowdermetallurgical and other metallureieal processes,-

Afterthese effects of theinvention are obtained,A thev reaction betweenthe nitric acid and powdery metal particles is interrupted quickly inQrdertoavoid losseso the metall substance by the.l continued attack;thereon of the concentrated nitric acid. To. this end the nitric acidcan be removed immediately aiter the desired effect of the, inyention isobtained., for instance by sepaeantinsthelatter Orrin a eentrifueineprocess.

Another and more convenient wer, Dertienlarly sul blefor messproduction, consiste in iow be described with reference to Fig. l and inits application to magnesium powder of any origin, although it should beunderstood that the same process can be applied to powder of any othermetal or to a mixture in desired and predetermined ratio of powders ofdifferent kinds of metals.

In a tank or container I a suspension of magnesium powder in anysuitable vehicle or suspension medium is prepared. Water is preferred assuch suspension medium. To this end, magnesium powder I4 is fedcontinuously from hop-- per I5 into tank IO at a predeterminable rate oramount per unit of time, which can be adjusted for instance byregulating the speed of a rotating feeder I6 arranged near the bottomand outlet oi hopper I5. Water is supplied to tank I 0 through pipe I?at a predeterminable rate perv time unit adjusted by adjustable valve29. Thereby any desired ratio of magnesium powder and water assuspension medium can be adjusted and maintained during operation, and asuiicient supply of that suspension provided. In order to suspend themagnesium powder in the water and maintain it suspended therein, air canbe injected into the water within tank lil as by an air supply lineindicated by the dotted lines l3-I, or mechanical stirring meansprovided as indicated by stirl rer II on shaft i2 of an electric motorI3.

The suspension or slurry thus formed is withdrawn from tank IU throughpipe I8 into reaction chamber I9 at a predeterminable rate which can beadjusted by any suitable means, such as adjustable valve 2I.

Tank I5 is arranged at a suitable highe'i` level than reaction chamberi9, so as to secure by action of gravity the desired flow of thesuspension or slurry into the reaction chamber, and for other purposesto be explained hereinafter. A supply of nitric acid of suitable highconcentration is maintained in tank 22, and a Ineasured quantity pertime unit of the acid is withdrawn from tank 22 through valve 23, pipe24 and another adjustable valve 25 into nozzle 26 arranged withinreaction chamber I9 so that the discharge end 2 of nozzle 28 is at alower level than the discharge aperture of pipe I8 into chamber I9.Nozzle 2t is of any suitable and known type and, if desired, it may beof a known type,

not shown, arranged to impart to the jet or spray of acid a rotationmovement. More than one nozzle or similar discharge devices can be used.

Upon proper adjustment of the adjustable valves 2| and 25 and afteropening' of valve 23, the suspension o1` slurry continuously formed intank il] and the highly concentrated nitric acid continuously suppliedto tank 22 are discharged and injected, respectively, into chamber I9 atpredeterminable and adjustable rates. The concentrated nitric acidinjected into the chamber space admixes rapidly with the watery slurryor suspension of magnesium powder simultaneously discharged into chamberi9. The concentration of the injected acid is such that its slightdilution by the suspension-water does not inhibit or substantiallykreduce its instantaneous and vehement reaction with the magnesiumpowder. Nitrogen-oxide gases develop turbulently by this reaction buttheir escape through either pipe I8 or 24 is prevented by thesufliciently high` pressure in the slurry and acid, respectively,flowing through those pipes and due to the arrangement of tanks i0 and22 at suitable higher levels than that of reaction chamber I9.

Considerable heat may be developed by-'this 4 reaction, depending on therelative amounts of acid and magnesium powder, and cooling means can beprovided in such case, such as a jacket 59 around chamber I9 throughwhich cooling water is passed in the manner indicated in the drawing.

While the powdery particles and acid travel downwardly through the spaceof chamber I9,

"the turbulent nitrogen-oxide gases developing during the reactionsecure the contact of all the particle surfaces by the acid, and theremoval of oxide and nitride lms in those surfaces and the cleaning ofthe latter is therefore completed after the particles and acid havetravelled a certain path or distance downwardly below the discharge end27 of nozzle 26. In order to interrupt the reaction at this point and toavoid losses of magnesium metal, the mixture of the powder and acidcould be discharged into a relatively large pool of continuouslyreplenished water which dilutes the acid sufficiently, or the powder andacid could be centrifugally separated in any known way. In Fig.` 1, atthe distance below nozzle 26 where the reaction accordingI to theinvention is completed, another nozzle 28 is arranged within chamber I9.Nozzle 28 discharges downwardly and is connected with pipe 29 passingthrough the wall-of chamber VI9 to the outside. Water is suppliedthrough pipe 29 from a source not shown, of a predeterminable quantityper time unit adjusted by adjustable Valve 30. Nozzle 23 can be of anysuitable type and supplies water in suiiiciently large quantity so thatupon itsvrapid and intimate admixture with the nitric acid arrivingbelow nozzle 28, the acid is diluted suiciently so'that its reactionwith the magnesium powder is linterrupted or at least the velocity ofthat reaction is reduced sufficiently so that no objectionable losses ofmagnesium occur thereafter.

It will be appreciated that the distance between nozzles 26 and 28depends on the now rate and concentration of the suspension or slurry ofmagnesium and on the flow rate and concentration of the nitric acid,Vall of which are adjustable at the start of and during operation in themanner hereinbefore described. The optimum distance between thosenozzlesv is chosen so as to cause complete removal of objectionablesurface lms from the metal particles. No general rule can be given forthis purpose because of the varying factors stated and the further` factthat average particle size and kind of the metal powder or mixture ofsuch powders comes into play too. However, after such optimum distancehas once been established for a given kind of powder or mixture ofpowders, continuous operation in the chamber I9 in mass production canbe maintained by adjusting the speed of feeder I6, `the amount of wateradmixed thereby by means of adjustable valve 20, the amount of thesuspension or slurry discharged into chamber I9 by means of adjustablevalve 2 I, and the amount of concentrated nitric acid injected throughnozzle 26 by means of adjustable valve 25. Thereby unavoidablevariations of the average particle size and other properties ofcommercially obtainable metal powders can be taken care of duringoperation without changing the distance between nozzles 26 and 28 or ofother dimensions and ar- Y rangements of the reaction chamber I9.

The rapid or almost instantaneous admixture of the diluting water andnitric acid is enhanced by the nitrogen-oxide gases previously developedwithin chamber I9 which are forced to escape -acecica thrmighthe loweropcnend of the chamber. This end is arranged within a pct 5i the bottom-of which is at some distance below the open end o! chamber I! and theside wall of which fis Asimceti from and extends considerably above thatend. Asa consequence, the mixture of cleaned magnesium powder anddiluted nitric iacid will collect in pot 5i to the level of `its upper`rim and thereby form a seal for the lower open end o! the chamber. Thenitrogen-oxide gases can penetrate that seal because the pressure in theliquid `c i'ilurnri between the upper end of the side wall of pot "5iand the lower end of chamber i3 is far smaller than the pressure inpipes i8 and 24, respectively. Pot V5i is illled almost immediatelyafter operation 'has started, andthe connuously supplied mixture of'magnesium powvder and diluted nitric acid ilows over the upper rim ofpot 5I into a settling tank 3i. The cleaned magnesium powder in theoverowing mixture will always be covered vfully by the lliquidconsisting of nitric acid and water, and detrimental access of air tothose particles is thereby prevented.

The slurry of magnesium particles and highly diluted nitric acidcollects in tank 3i Since the nitrogen gases escaped previously in themanner hereinbefore described, this mixture cornes to a rest within tank3| and segregates. The heavier magnesium powder sinks toward andaccumulates upon the downwardly tapering bottom 33 of tank 31 up tolevel 34, whereas the diluted nitric acid forms a liquid layer on top ofthe settled magnesium powder to the level 32. Inside tank 3i and Vnearits upper open end, a trough 35 is arranged acting as an overflow forthe diluted nitric acid which is withdrawn from trough 35 in the mannershovm.

The magnesium powder collected above bottom 33 forms a slurry stilladmixed with diluted nitric acid; itis withdrawn from tank 33 at propertime intervals, as explained later, through valve 36 into a vacuum lter,such asa washing and ltering vessel or tank 31 provided with adownwardly tapering bottom 33. A filter 33 is arranged in vessel 31above and spaced from bottom 38. Clean washing water is poured orsprinkled through pipe 40 upon the slurry deposited on lter 33 andaccumulates above the powder to the level 44 of an overflow (trough) 4ifrom which it is withdrawn in the manner shown. Vessel 31 communicatesthrough an aperture in its bottom 38 and connecting pipe with anotherconilned space 42 thereunder in which a vacuum is produced andmaintained by pipe 43 connected with a vacuum pump (not shown). Therebya great part of the liquid consisting of washing water, admixed with themagnesium powder above filter 33 is sucked away through filter 39 andthe magnesium powder slurry more concentrated to form a. cake whichstill contains but little liquid. Any access of air to powdery particlesof the cake is prevented by the layer of washing Water collected aboveit. Any liquid collecting in vessel 42 can be withdrawn therefromthrough a .pipe and valve 50. After a lter cake of sufficient height hasbeen built up, the valve 3i is closed and the filter cake allowed toremain until the water entering through the pipe 40 has thoroughlycleaned the surfaces of the metal particles in the cake. The filter cakeis then removed from vessel 31 and transferred into a dryer. The valve36 may then be opened to allow another quantity of slurry `to enter thevessel 31 to form a new filter cake. The extraction of the water stilicontainedin the .filter cake can be accomplished by treating it with `asuitable `extraction medium, such as acetone. The .extraction of thewater and the drying of the lter cake may 'be eifected in Ia vacuumdryer of any known suitable type. Whichever lrind of dryer and means fortransferring "the cake into it are used, it is essential that themagnesium particles of the cake are wet luntil they are placed 'into thedryer, so that access of air to the magnesium particles is preventedwhich, before 'the latter are completely dried, could 'react with theparticle surfaces `in `spite o'f their thorough ycleanliless.

Fig. 2 indicates, by way of example, one form of a vacuum dryer having arevolvable drum '46 in which'vacuumiisproducedby means of pipe 4Bprojecting into the vinterior ofthe drum land connected with a Vacuumpump (not shown). A mild heat, up to about 50 C.,'isproduced within thedrum, for instance, by means of gas flames 4B of a gas burner 41.Thereby the .strong cooling effect resulting from 'the evaporation invacuo of the liquid adhering to the magnesium powder can be compensatedto desired extent. The vacuum produced within vdrum 4t should be oi a.pressure equall'ing or below 4to 5 millimeters mercury column absolute.While the filter cake is being dried, it breaks up into powder; afterthe powder vis completely dried, the vacuum can be released and thepowder removed from the drum. The completely dry powder is clean ,andshiny, and its surface layers are resistant to contamination in theaforementioned sense for a limited period of time. 'The powder cantherefore be manipulated, such as compacted for metallurgicalpurposes,without the danger of the formation of objectionable oxide or nitridelms thereon.

lf the dry powder is to be stored or shipped, an inert gas can beadmitted into drum 46 immediately after the vacuum has been released,for instance, very dry hydrogen or an inert gas like helium, argon orneon. The 'inert gas may be supplied through the same tube 45 throughwhich the drum 46 is evacuated. In certain cases and if the interior ofthe drum and the powder ltherein have been permitted to coo1 to roomtemperature while the vacuum is on, dry nitrogen can be used asprotective gas and admitted into the drum after the vacuum has beenreleased. The powder and inert gas of the types mentioned can be filledinto containers or cans which are air-tightly sealed thereafter.

if an inert gas, such as helium, etc., has been admitted into the drum,a layer of that gas may be adsorbed by the dry powder particles whichadditionally protects the latter against Voxidation for some timethereafter.

use of a protective gas, such las very dry hydrogen of a dew point farbelow minus 40 C., or helium, argon or neon, even such superficialoxidation or nitrid'ing can be prevented. Since compacts of high densitycan be produced of 'the metals herein concerned by relatively lowcompaction pressures, no danger of undue absorption of the protectivegas or penetration of oxygen or nitrogen into the interior of thecompact exists. Hence the final product obtained Aby sintering Will beof greatest purity, homogeneity and density and therefore exhibitunusual or heretofore unknown physical properties. The same advantagesare obtained if the powder processed according to the invention isadm'ixed with other clean powders in order to obtain from the powderymixture a desired metal alloy, or if a mixture of those powders isprocessed, bodies or even ingots compacted and sintered therefrom willexhibit outstanding properties due to their homogeneity, density andpurity. It is well known in the art that casting of magnesium, aluminum,or alloys of those metals requires a large riser because of the highshrinkage of those metals and alloys, and their production is thereforeconnected with extensive melting losses. The control of grain size visequally difiicult in casting those metals and alloys, and the formationof blisters and blow holes therein is hard to avoid, if possible at all.The properties of the alloys depend greatly on the purity and exactratio of the alloying constituents therein which to obtain 'is extremelydiicult in casting processes. Therefore, if ready to use bodies, oringots for further working and shaping are made by powder metallurgicalprocesses from powder or powdery mixtures containing metal powderprocessed according to the invention, all these difficulties can beobviated. However, these aspects of the use of powders processedaccording to the invention do not form a subject matter thereof and arementioned only to illustrate the high utility and novel effects ofpowders processed according to the invention.

It should be understood that the invention is not limited to any of theembodiments herein exemplified and illustrated. Thus, for instance,instead of single nozzles 26 and 2B, any number of them can be arrangeddischarging at the same lor slightly different levels, and the supply tothem of the slurry and nitric acid, respectively, can be regulatedsimultaneously or independently. Instead of a vacuum lter as exempliedhereinbefore, any other type of filter can be used, such as a continuousworking filter. Whatever the implements and equipment used forpracticing the invention may be, their essential operation shouldconsist in reacting upon the metal powder or mixture of such powderswith nitric acid of suiiiciently high concentration so as to rapidlyremove from the powdery particles oxide and nitride films and also cleanthe surface layers of the particles; to interrupt this reactionimmediately after these effects are obtained in order to avoid losses ofmetal substance; to wash the cleaned powdery particles and to dry themin the absence of air, i. e. in a sufficiently high vacuum and/or in thepresence of dry and inert gas (e. g. helium), not shown; and to storethe processed dry powder, 'if required, in air-tight containers andadvantage- 'ously in the presence of an inert gas until the powder isfurther processed or used, particularly in a powder metallurgicalprocess.

What I claim is: l. The method of cleaning light powdery particles ofmetal selected from the class consisting `of aluminum and magnesium andhaving metal compound surface impurities which are removable by nitricacid, which comprises the steps -of reacting upon a uid mixturecontaining water and said powdery particles with concentrated nitricacid so as to remove metal compounds from the surfaces of said particlesand thus produce on said particles cleaned surface layers, effectivelyinterrupting immediately thereafter said reaction by effectivelyremoving the powdery particles from the action of the concentrated acid,and thereafter separating said particles from the resulting mixtureunder conditions excluding the access of air.

2. The method of cleaning light powdery particles of metal selected fromthe class consisting of aluminum and magnesium and having metal compoundsurface impurities which are removable by nitric acid, which comprisesthe steps of reacting upon a fluid mixture containing water and thepowdery particles with concentrated nitric acid so as to remove metalcompounds from their surfaces and thus produce cleaned surface layers,and effectively interrupting immediately thereafter said reaction bydiluting the acid and separating said particles from the resultantmixture under conditions excluding the access of air.

3. The method of cleaning light powdery particles of metal selected fromthe class consisting of aluminum and magnesium and having metal compoundsurface impurities which are removable by nitric acid, which comprisesthe steps of reacting upon a fluid mixture containing water and thepowdery particles with concentrated nitric acid so as to remove metalcompounds from their surfaces and thus produce cleaned surface layers,effectively interrupting immediately thereafter said reaction,thereafter adding water to the resulting mixture, and washing and dryingthe particles under conditions excluding the access of air.

4. The method of cleaning light powdery particles of metal selected fromthe class consisting of aluminum and magnesium and having metal compoundsurface impurities which are removable by nitric acid, which comprisesthe steps of reacting upon a mixture of water and the powdery particleswith concentrated nitric acid so as to remove metal compounds from thesurfaces and thus produce cleaned surface layers on said particles,effectively interrupting immediately thereafter said reaction,thereafter adding a washing agent to the resulting mixture and washingthe particles with said washing agent so as to protect said particlesagainst the access of air, and removing the washing agent and othermixture ingredients still adhering to the particles by treating themwith a volatile solvent medium.

5. The method of cleaning light powdery particles of metal selected fromthe class consisting of aluminum and magnesium and having metal compoundsurface impurities which are removable by nitric acid, which comprisesthe steps of reacting upon a fluid mixture containing water and thepowdery particles with concentrated nitric acid so as to remove metalcompounds from their surfaces and thus produce cleaned surface layers,effectively interrupting thereafter said reaction, thereafter adding awashing agent to the resulting mixture and washing the particles undercover of a washing agent on a vacuum lter, and drying the wet filtercake in a nonoxidizing atmosphere.

6. In a method as set forth in claim 5, drying the wet filter cake inthe presence of an inert gas.

7. In a method as set forth in claim 5, drying the wet filter cake inthe presence of an inert gna` selectedfrom group consisting of helium,mlm and neon.

8. In a method as setforth in claim 5, drying said filter cake in avacuum the pressure of which does not exceed about 5V millimetersmercury column absolute.

9. In a method as set forth in claim 5, drying the lter cake aty atemperature up to about 59 C;

10. The method. of cleaninglight powdery particles of metal selectedfrom the classv consisting of aluminum and magnesium vand havingmetalcompound surface impuritiesA which. are removable. by nitric acid,which comprises the steps 0f suspending the metal-powder in water so asto form a slurry, admixing with said slurry niacid of highconcentrationand in an amount sumcient to secure an intense reaction of said acidupon the metal particles and thereby to remove metal compound impuritieson the surfaces of said particles and to thus produce cleaned surfacelayers on said particles, and effectively interrupting immediatelythereafter said reaction by diluting said acid.

1l. The method of cleaning light powdery particles of metal selectedfrom the class consisting of aluminum and magnesium and having metalcompound surface impurities which are removable by nitric acid,which-comprises the steps of suspending the metal powder in water so asto form a slurry, admixi'ng with said slurry nitric acid of highconcentration andv in an amount mmcient to secure an intense reaction ofsaid acid upon the metal particles and thereby to remove metal compoundimpurities on the surfaces of said particlesv and to thus producecleaned surface layersr on said particles, effectively interruptingthereafter: said reaction by effectively removing said particles' fromthe aetion of said concentrated acid, substantially separating said acidand particles, washing the particles with and separatingr themthereafter from a washing agent, and drying the washed particles.

l2. In a method as set forth in claim 11, substantially separating theacid and particles by allowing the latter to settle as slurry, washingthe particles of said slurry with water and removing a substantialportion of said washing water by filtering, and thereafter drying theparticles, said settling, washing, filtering and drying performed underconditions excluding the access of air to the particles until they arecompletely dry.

13. The method of cleaning light powdery particles of metal selectedfrom the class consisting of aluminum and magnesium and having metalcompound surface impurities which are removable by nitric acid, whichcomprises, in a continuous process, the steps of supplying continuouslyat predetermined rates and mixing metal powder and water so as to form aslurry thereof, discharging said slurry into confined space, admixingconcentrated nitric acid with said slurry within said space so that theacid reacts upon the metal particles and removes from their surfacesmetal compound impurities and thus produces cleaned surface layers whilesaid particles and acid move downwardly through said space, dischargingwater into said space substantially at the place where said reaction hasjust been completed so as to dilute the acid and substantially reducethe velocity of said reaction, discharging the mixture of diluted acidand cleaned particles lfrom said ,space under conditions per- 10 mittinggases developing during said reaction` to escape, land thereaftersubjecting the resulting mixture to a treatment` including washing ofsaid particles and separation of said pas-ticlesfrom the resultingmixture under conditions excluding access of air to said particles.

14 In a method as set forth in claim i3, suppiying said initialsuspension or slurry and said concentrated nitric acid from levelsconsiderably higherthan the levels at which said suspension and acid aredischarged into said confined space,

i5,. In a method as set forth in claim 13, adjusting independently thequantities per time unit of metal powder and suspension water to formsaid initial slurry and the quantity per time unit of said initialslurry fed into said conned spa-ce, and also adjusting independently thequantity per time unit of concentrated nitric acid. supplied to saidconfined space so as f to obtain and maintain a predetermined optimumratio of said acid and powdery particles for saidl reaction.

16. In a method as set forth in. claim 13, ab stracting heat from` saidconfined space While i said reaction proceeds..

17. In a method as set forth in claim 13, admixing by injection underpressure said nitric acid with. said initial suspension or slurry insaid conned space.

1.8. In a method as set forth in claim 13,. admixing by injection underpressure the diluting water with said mixture of acid and slurry withinsaid confined space after said reaction has been. completed.

-l9. In a method as set forth in claim 1.3, said. treatment includingcollecting said mixture and allowing it to rest so that saidv particlesseparato substantially from` the diluted acid and form an intermediateslurry, subjecting said intermediate slurry to a washing treatment,increasing the concentration of the washed. slurry, and treatit toobtain ciean dry metal powder, said treatment including withdrawing atpredetermined intervals the settled intermediate slurry for subsequentwashing and drying.

20. In a method as set forth in claim 13 said treatment includingcollecting said mixture and allowing it to rest so that said particlesseparate substantially from the diluted acid and form an intermediateslurry, subjecting said intermediate slurry to a washing treatment,increasing the concentration of the washed slurry, and treating it toobtain clean dry metal, said treatment also including, increasing theconcentration of the withdrawn intermediate slurry by a vacuum filtertreatment.

21. In a method as set forth in claim 13 said treatment includingcollecting said mixture and allowing it to rest so that said particlesseparate substantially from the diluted acid and form an intermediateslurry, subjecting said intermediate slurry to a washing treatment,increasing the concentration of the washed slurry, and treating it toobtain clean dry metal, said treatment also including filling the dried,clean metal powder immediately upon completion of said treatment into acontainer and thereafter sealing the latter air-tightly.

22. In a method as set forth in claim 13 said treatment includingcollecting said mixture and allowing it to rest so that said particlesseparate substantially from the diluted acid and form an intermediateslurry, subjecting said intermediate slurry to a washing treatment,increasing the concentration of the Washed slurry, and

treating it to obtain clean dry metal, said treatment also includingfilling the dried, clean metal powder immediately upon completion ofsaid treatment into a container in the presence of an inert gas, andthereafter sealing said container air-tightly.

23. The method of cleaning light powdery particles of metal selectedfrom the class consisting of aluminum and magnesium and having metalcompound surface impurities which are removable by nitric acid, whichcomprises the steps of reacting upon a fluid mixture containing waterand 'said powdery particles with concentrated nitric acid so as toremove metal compounds of the surfaces of said particles and thusproduce on said particles cleaned surface layers, effectivelyinterrupting immediately thereafter said reaction by centrifugallyseparating said particles from said acid, and thereafter separating saidparticles from the resultingmixture under conditions excluding theaccess of air.

24. In a method as set forth in claim 5, drying the wet filter cakeunder exclusion of air.

25. The method of cleaning light powdery particles of metal selectedfrom the class consisting of aluminum and magnesium and having metalcompound surface impurities which are removable by nitric acid, whichcomprises the steps of suspending the metal powder in Water so as toform a slurry, admixing with said slurry nitric acid of highconcentration and in an amount suflicient to secure an intense reactionof said acid upon the metal particles and thereby to remove metalcompounds formed on the surfaces of said particles to thus producecleaned surface layers on said particles, and effectively interruptingimmediately thereafter said reaction by effectively removing the powderyparticles from the action of the concentrated acid.

26. The method of cleaning light powdery particles of metal selectedfrom the class consisting of aluminum and magnesium and having metalcompound surface impurities which are removable by nitric acid, Whichcomprises the steps of suspending the metal powder in water so as toform a slurry, admixing with said slurry nitric acid of highconcentration and in an amount sufficient to secure an intense reactionof said acid upon the metal particles and thereby to remove metalcompounds formed on the surface of said particles for producing cleanedsurface layers on said particles, and effectively interruptingimmediately thereafter said reaction by centrifugally separating saidparticles from said acid.

27. In a method as set forth in claim l2, effectively interrupting saidreaction by diluting said acid.

FRITZ J. HANSGIRG,

References Cited in the le of this patent v UNITED STATES PATENTSGermany Dec. 13, 1928

1. THE METHOD OF CLEANING LIGHT POWDERY PARTICLES OF METAL SELECTED FROMTHE CLASS CONSISTING OF ALUMINUM AND MAGNESIUM AND HAVING METAL COMPOUNDSURFACE IMPURITIES WHICH ARE REMOVABLE BY NITRIC ACID, WHICH COMPRISESTHE STEPS OF REACTING UPON A FLUID MIXTURE CONTAINING WATER AND SAIDPOWDERY PARTICLES WITH CONCENTRATED NITRIC ACID SO AS TO REMOVE METALCOMPOUNDS FROM THE SURFACES OF SAID PARTICLES AND THUS PRODUCE ON SAIDPARTICLES CLEANED SURFACE LAYERS, EFFECTIVELY INTERRUPTING IMMEDIATELYTHEREAFTER SAID REACTION BY EFFECTIVELY REMOVING THE POWDERY PARTICLESFROM THE ACTION OF THE CONCENTRATED ACID, AND THEREAFTER SEPARATING SAIDPARTICLES FROM THE RESULTING MIXTURE UNDER CONDITIONS EXCLUDING THEACCESS OF AIR.